"In negative relationships, however, the situation is reversed. Bad
behavior is considered the norm."

Basically, healthy relationships are
optimistic; if a couple argues, they assume they did it because they had
an off day. In negative relationships, people just assume their partner
is mean or selfish.

Fry's formula takes mood, expectations, and
influence on each other into account. She says the healthiest
relationships are one with a high "negativity threshold," where couples
are allowed to complain and voice their concerns without getting mad at
each other. Then they're able to work together to fix these concerns.